This invention relates in general to a high side switch for a field-effect transistor. In particular, this invention relates to controlling a voltage to a load through such a high side switch.
A transistor such as, e.g., a field-effect transistor (FET) may be used for amplifying or switching electronic signals. When used as a switch between a positive power supply and a load, a high side driver circuit is typically provided. The high side driver circuit contains control and power sub-circuits to operate the gate of the FET. The high side driver and the FET may optionally be included in one package as a high side switch integrated circuit (IC).
A high side switch may use an n-channel metal oxide semiconductor FET (MOSFET) or a p-channel MOSFET. An n-channel MOSFET may be preferable due to having a lower ON-resistance and better efficiency. An n-channel MOSFET switch normally requires a gate voltage higher than the voltage of the power supply to turn the switch on. The increased voltage may be provided by an internal charge pump included with the high side driver.
FIG. 1a shows a schematic representation of a typical prior art n-channel high side IC switch. The n-channel high side switch includes an input connected to drain D, which may also be the source of power for the control section of the switch. The output is controlled by a switching transistor, which may be an n-type MOSFET. A gate G is controlled by logic which is discrete or integrated in a controller IC. The logic may be applied to the gate using a boosted voltage from a charge pump. The charge pump may draw a low current from the VCC supply, which may be several milliamps.
As shown in FIG. 1b, the charge pump may stop operating if the voltage supplied drops below an undervoltage threshold, shown at time t1. This may in turn cause the switch to turn off with the effect that the supplied load is switched off, even when the voltage drop occurs for just some milliseconds, such as shown in FIG. 1c, between time t1 and time t2. This may occur when the supply voltage drops due to, for example, increased drain on a battery that provides the supply voltage.